Whether deafness occurs as a result of mechanical trauma, noise over-stimulation, or aging (presbyacusis), the mechanisms regulating central changes resulting from this deficit are poorly understood. Thus, our long- term goal is to understand the role of dopaminergic circuits, within the central auditory system and how they change following deafness. Dopamine (DA) is an inhibitory neurotransmitter that modulates the action of amino acid neurotransmitters. In the frontal cortex DA affects cognition with through facilitation of glutamatergic effects. Dopamine can also affect the release of co-localized GABA and glutamate by binding receptors at pre-synaptic sites. As a first step towards achieving our goal, the studies we have proposed in Specific Aims 1-3 use tract tracers to identify and characterize three dopaminergic pathways originating in the external cortex of the inferior colliculus (ICex), central nucleus of the inferior colliculus (ICC), and nuclei of the lateral lemniscus (NIL). To determine whether these pathways are dopaminergic, we will co-immunolabel for tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of dopamine, and tract tracers. We will also use the dopaminergic neurotoxin MPTP to confirm the presence of DA in midbrain circuits. In other systems DA is often co-localized with other neurotransmitters, therefore, we will examine whether dopaminergic neurons in these brain regions co-localize with neurotransmitters traditionally associated with auditory midbrain regions (GABA, glycine and glutamate). There are deafness related decreases in the gene expression of TH, (Holt et al., 2005) in the IC. Dopamine immunolabeled neurons have been identified in the IC and lateral lemniscus and there is a dramatic decrease in dopaminergic neurons in these brain regions following deafness (Tong et al., 2005). Our final specific aim then addresses the relationship between deafness related decreases in TH levels and changes in the production of other co-localized transmitters. Characterizing the circuitry and neurochemistry of these pathways allows for future studies that examine the function of specific dopaminergic pathways in this system. The current application paves the way for functional studies, developing the use of stereotaxic MPTP injections to lesion DA pathways. Ultimately, understanding DA pathways in the auditory system may lead to the development of novel mechanisms for modulation neuronal activity. Identifying key neurotransmitters within the central auditory system leads to understanding pathways into which they are integrated, their function, and ultimately help us devise methods to manipulate these key players to provide the best possible hearing outcomes for those with cochlear prostheses or those individuals being treated for presbyacusis. [unreadable] [unreadable] [unreadable]